Using density functional theory calculations with an on-site Hubbard term (DFT+U), we explore the effect of surface termination and cation substitution on the performance of the Co_xNi_1–xFe₂O₄(001) surface (x = 0.0, 0.5, 1.0) as an anode material in the oxygen evolution reaction (OER). Different reaction sites (Fe, Co, Ni, and an oxygen vacancy) were investigated at three terminations: the B-layer with octahedrally coordinated Co/Ni and with an additional half and full monolayer of Fe (0.5A and A-layer, respectively). Ni substitution with an equal concentration of Co and Ni (x = 0.5) reduces the overpotential over the end members for the majority of reaction sites. Surface Co cations are identified as the active sites and the ones at the A-layer termination for x = 0.5 exhibit one of the lowest theoretically reported overpotentials of 0.26 V. The effect of the additional iron layer on the active site modification is 2-fold: analysis of the electronic properties and spin densities indicates that the additional Fe layer stabilizes a bulk-like oxidation state of +2 for Co and Ni at the A-layer termination, whereas at the B-layer termination, they are oxidized to 3+. Moreover, the unusual relaxation pattern enables the formation of a hydrogen bond of the OOH intermediate to a neighboring surface oxygen that lowers the reaction free energy of this formerly rate-limiting step, leading to a deviation from the scaling relationship and almost equidistant reaction free-energy steps of intermediates. This renders an example of how a selective surface modification can result in a significant improvement of OER performance.

中文翻译：

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用于水氧化的Co _x Ni _{1– x} Fe ₂ O ₄（001）表面的表面终止和活性控制：从DFT + U计算得出的见解

使用现场哈伯德项（DFT + U）的密度泛函理论计算，我们探索了表面终止和阳离子取代对Co _x Ni _{1– x} Fe ₂ O ₄（001）表面性能的影响（x = 0.0 ，0.5、1.0）作为氧气逸出反应（OER）中的阳极材料。在三个端点处研究了不同的反应位点（Fe，Co，Ni和氧空位）：具有八面体配位的Co / Ni的B层以及另外一半和完全的Fe单层（分别为0.5A和A层） ）。用等浓度的Co和Ni替代Ni（x= 0.5）减少了大多数反应位点的末端成员上的过电势。表面Co阳离子被标识为活性位点，在x的A层终端处被标识为= 0.5表示理论上最低的过电势之一，为0.26V。附加铁层对活性位点修饰的影响是2倍：对电子性能和自旋密度的分析表明，附加铁层稳定了块状Co和Ni在A层终端处的氧化态为+2，而在B层终端处，它们被氧化为3+。此外，异常的弛豫模式使OOH中间体与相邻的表面氧形成氢键，从而降低了该先前限速步骤的反应自由能，从而导致与比例关系发生偏离，并且反应自由度几乎等距。中间体的能级。